An atomic force microscope (AFM) is a comparatively high-resolution type of scanning probe microscope. With demonstrated resolution of fractions of a nanometer, AFMs promise resolution more than 1000 times greater than the optical diffraction limit.
Many known AFMs include a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into contact with a sample surface, forces between the tip and the sample lead to a deflection of the cantilever. One or more of a variety of forces are measured via the deflection of the cantilevered probe tip. These include mechanical forces and electrostatic and magnetostatic forces, to name only a few.
Typically, the deflection of the cantilevered probe tip is measured using laser spot reflected from the top of the cantilever and onto an optical detector. Other methods that are used include optical interferometry and piezoresistive AFM cantilever sensing.
One component of AFM instruments is the actuator that maintains the angular deflection of the tip that scans the surface of the sample. Most AFM instruments use three orthonormal axes to image the sample. The first two axes (e.g., X and Y axes) are driven to raster-scan the surface area of the sample with respect to the tip with typical ranges of 100 μm in each direction. The third axis (e.g., Z axis) drives the tip orthogonally to X and Y for tracking the topography of the surface.
Generally, the actuator for Z axis motion of the tip to maintain a near-constant deflection requires a comparatively smaller range of motion (e.g., approximately 1 μm (or less) to approximately 10 μm). However, as the requirement of scan speeds of AFMs increases, the actuator for Z axis motion must respond comparatively quickly to variations in the surface topography. In a contact-mode AFM, a feedback loop is provided to maintain the tip of a cantilever in contact with a surface. At high scan speeds and low force setpoints, however, the tip can detach from the surface, for example if the tip passes over a comparatively large depression in the surface. A cantilever that is off the surface of the sample (i.e., detached) can resonate at its natural resonant frequency. As the scan rate increases, the bandwidth of the controller must commensurately increase. This off-surface resonance can fall within the bandwidth of the feedback loop, which can amplify the resonance and cause the system to become unstable. Ultimately, this can damage samples and reduce the resolution of the images from the AFM.
There is a need, therefore, for a controller for an AFM and an AFM system that overcomes at least the shortcomings of known controllers discussed above.